Weedle got that quote from the official interview with the storyboard director / director of the anime on PokeBeach. There are actually two interview with that guy. He later said Misty will never return to the main cast, but will continue to make cameo appearances like she has in the past. He also hinted it might be at some battling competition to get her into Sinnoh.
Yeah, though considering the facts that he was demoted to being a Background Designer rather than a Director, he's really in no position to say that for me (and, basically, anyone else, for that matter) to take it seriously (Plus, it was something even WPM stated, and I really hate it when people like "Ling Ling" think I'm not being serious and demand proof.), The same goes to stuff like Ash not leaving, and Ash's old Pokemon returning (IE, Pidgeot, Butterfree, Lapras, Primeape, etc, etc.).
Plus, he didn't even know Platinum was going to be out (I can understand his denying Arceus's existance, since it's not even supposed to be officially revealed yet, but Platinum has been known by nearly everyone and their brother to be in development, and they revealed a bit too much as it is to deny it's existance, and yet he legitimately didn't know.), so why should we take him seriously.
I mean, to quote Claire Bennet from Heroes:
"The future is not yet set in stone."
Of course, even IF he was still the director, considering how the Japanese writers officially "Confirmed" that we would meet Ash's father in Season 5 (which turned out not to be. Officially confirmed, my foot.), I can't say we should trust him in regards to future events, as he might end up lying. I mean, I can at least trust the part in regards to his reasons for replacing Misty, his creating May and Dawn, his creation of Tracey/removal of Brock, the GS ball, I can trust, as those are "past" events where he was still the director, and that he at least gave an answer and took a significant amount of time to be validated.